Castration resistant prostate cancer (CRPC) is incurable as the current anti-CRPC treatments do not extend life beyond a few months. Cytochrome P450 3A4 (3A4) is an enzyme that metabolizes and inactivates a number of xenobiotics, including anti-CRPC drugs Abiraterone and Docetaxel. Although, such metabolism usually occurs in the liver, preliminary data show elevated expression of 3A4 (mRNA and protein) and activity in prostate cancer (PCa) cells, when compared to normal prostate cells. Further, a dietary supplement, 4-methylumbelliferone (MU) downregulates 3A4 expression in PCa cells; consequently MU downregulates 3A4 activity and enhances cytotoxic effects of anti-CRPC drugs. Interestingly, a 3A4-variant (3A41B) generated by DNA polymorphism correlates with PCa metastasis and has high promoter activity. This exploratory project is designed to address an understudied area in PCa biology: is elevated expression of 3A4 in PCa cells a mechanism for their reduced sensitivity to anti-CRPC agents? Further, does downregulation of 3A4 expression in PCa cells, by a non-toxic dietary supplement, enhance the sensitivity of PCa cells to anti- CRPC agents? The role of 3A4 in PCa cells will be examined by determining whether modulation of 3A4 levels (over expression and silencing) and MU treatment alters the sensitivity of PCa cells to anti-CRPC agents. Furthermore, does 3A4 expressed in PCa cells inactivate anti-CRPC drugs and whether MU prevents this inactivation? Studies will also investigate the mechanism by which MU downregulates 3A4 levels; specifically, whether MU increases the degradation of 3A4 mRNA or transcriptionally silences both 3A4 wild type and variant 3A41B promoters. Efficacy of anti-CRPC agents, either alone or in combination with MU will be tested to inhibit the growth and progression of castration-resistant prostate tumors in xenograft models. PCa cells used in the xenograft will be those in which 3A4 expression is either up- or down-regulated. Impact: This exploratory project should reveal the role of 3A4 expressed in PCa cells in modulating the response of PCa cells to anti-CRPC agents. Furthermore, does combination with a non-toxic dietary supplement that specifically targets 3A4 expression enhances the sensitivity of PCa cells to anti-CRPC agents. If successful, the project has potential for clinical translation, in which based on the 3A4 levels in tumors a 3A4 inhibitor could be added to improve response to anti-CRPC treatments.